Primary leiomyosarcoma of the kidney is a rare entity that has not been well characterized. We retrieved 27 cases of primary renal leiomyosarcomas diagnosed at 3 institutions between 1986 and 2009. Mean patient age at diagnosis was 58.5 years (range 22 to 85), and 59% were female. Mean tumor size was 13.4 cm (range 4 to 26), and 59% of the tumors were identified in the right kidney. Detailed histologic examination was possible for 24 of the cases. Average mitotic count per 10 high-power fields was 11.1 (range 0 to 50), and the average extent of necrosis was 21% (range 0 to 60). Cellular pleomorphism was classified as either focal (n = 13) or extensive (n = 11) and graded as mild (n = 3), moderate (n = 7) or severe (n = 14). Tumors were either grade 2 (n = 12) or grade 3 (n = 12) using the French Federation of Cancer Centers System. Direct extension beyond the kidney capsule was identified in 55% of the cases, and lymphovascular invasion was identified in 26%. Clinical follow-up information was available for 20 of the cases, and patients were followed for an average of 2.8 years (range 0.25 to 9). Distant metastases were identified in 90% of the patients, and 75% eventually died from their tumor's burden. In conclusion, primary renal leiomyosarcomas have a grim prognosis regardless of the underlying histology.